Electronic circuits often require timing pulses of a selected, stable frequency to control various functions. Phase-locked loops are sometimes utilized to provide such timing pulses. However, some disadvantages are associated with phase-locked loops. For example, phase-locked loops typically utilize a voltage-controlled oscillator (VCO) in the feedback path. Various limitations are associated with analog voltage-controlled oscillators. Nonlinear effects in MOS transistors of the VCO result in a highly nonlinear transformation of voltage to frequency by the VCO. Another disadvantage of utilizing a VCO is a substantial relative intrinsic delay in the response of the VCO.
In addition, in a conventional phase-locked loop system, a loop filter is typically used to obtain frequency stability in the feedback loop. The loop filter typically includes two capacitors and a resistor which are external to the integrated circuit. An integrated circuit normally produces some internal digital switching noise which induce noise signals on VDD and VSS power supply lines. Because the ground reference potential internal to the integrated circuit includes some noise, the ground potential internal to the integrated circuit has a different potential than the ground reference external to the integrated circuit. This voltage difference between the ground potential internal to the integrated circuit and the ground potential of the loop filter components outside the integrated circuit unavoidably introduces some noise, called phase jitter, on the output of the phase-locked loop.
In many applications, performance degradation arising from phase jitter and nonlinear behavior of the voltage-controlled oscillator is too great to overcome using a phase-locked loop circuit. Therefore a new approach is sought.